1. Field of the Invention
This invention relates to methods for testing semiconductors, and more particularly, to a method of hot-carrier reliability test for evaluating the lifetime of a plurality of metal-oxide semiconductor (MOS) transistors in a semiconductor wafer under the influence of hot-carrier effects. The method is based on wafer-level rather than package-level, allowing the hot-carrier reliability test to be performed out with increased efficiency and cost-effectiveness.
2. Description of Related Art
A hot-carrier reliability test is a test procedure for evaluating the lifetime of MOS transistors formed in finished products of semiconductor devices under the influence of the hot-carrier effects. One conventional method for hot-carrier reliability test is first to divide the MOS transistors under testing into at least three groups, and then apply different drain voltages to the different groups of the MOS transistors for multi-device stress.
To achieve the multi-device stress, the testing circuit should be devised in such a manner as to be able to supply a total of at least eight testing voltages, including three drain voltages, three gate voltages, one substrate voltage, and one source voltage. This number of the testing voltages is often considered quite laborious to connect to the transistors undergoing the test. The testing procedure is therefore quite time-consuming and costly to perform.
Moreover, the conventional method is takes place at the package-level, which means that the hot-carrier reliability test is conducted after the semiconductor wafer is packaged. In this way, the testing voltages are connected via the output pads on the IC package. One major drawback to the package-level method, however, is that the packaging of the semiconductor wafer usually requires several weeks to complete. Therefore, the hot-carrier reliability test has to wait for several weeks to be performed after the fabrication of the semiconductor wafer is completed. This significantly delays the delivery of the products of the semiconductor devices and thus makes them less competitive in the market.